Transport Magnetostratigraphy: Preliminary Results from the Cretaceous Nanaimo Group
Housen, B (1)
(1) Geology Department, Western Washington University, Bellingham, WA 98225-9080
Abstract:
The Nanaimo Group is a series of Upper Cretaceous clastic sediment formations deposited in a shallow-to-deep marine basin overlapping the Coast Mountains orogen and the Insular Superterrane, both elements of Baja British Columbia. The age range of the Nanaimo Group is significant in that it encompasses nearly the entire period of northward translation of the Insular and Coast Mountains from a position at ca. 25 N to its present position relative to NorthAmerica. Sediments in this basin thus have the potential to preserve a record of changing sediment source as Baja BC was translated along the margin of North America. If these sediments also preserve a depositional-age paleomagnetic signal, a magnetostratigraphy from the Nanaimo Group would record both changes in paleolatitude and provide detailed age control for the sediments. With such a detailed record of the pole-ward component of translation, an improved comparison between possible translation velocities predicted by plate tectonic models and those derived from paleomagnetic data could be made.
Several obstacles exist for utilization of a transport magnetostratigraphy from the Nanaimo Group sediments. Because a depositional-age remanence would be a detrital remanence, inclination shallowing due to compaction is clearly possible. In fact, because the expected translation during sediment deposition is pole-ward, the shallowest inclinations should be found in the oldest (and most deeply buried) sediments. A correlation between shallow inclinations and burial depth is also expected if inclination shallowing is a significant problem. Following the publication of work done by Peter Ward and colleagues on Nanaimo sediments, a geophysics seminar class revisited Nanaimo outcrops sampled by Myrl Beck and one of his students in the early 1970s. We have obtained preliminary results from 5 sites taken from folded outcrops of the Haslam Formation of the Nanaimo Group on Orcas Island. Using a combination of demagnetization techniques and magnetic anisotropy methods we will assess the possibility of inclination shallowing on the characteristic remanence of these sediments. Pending the outcome of additional laboratory work, our preliminary data indicate that the remanence of these rocks is pre-folding in age (tilt-corrected mean D = 39, I = 39, alpha95 = 13, k = 54), and there is no observed correlation between magnetic anisotropy and inclination. These results are consistent with the published results of Ward et al., 1997, in that both studies indicate that the Nanaimo sediments were deposited at a latitude of 22-25 N.
